The present invention pertains generally to fluid control devices and, more particularly, to a throttle device for controlling and throttling high fluid pressures in equipment utilized in the oil, gas, and chemical industries as well as in power plants.
The prior art includes many fluid control devices that are configured to control the velocity of the flowing fluid. These prior art devices are typically comprised of a hollow cylinder with an arrangement of orifices through which high-pressure fluid flows. The orifices may be formed by drilling or machining of the hollow cylinder. The passage of the high-pressure fluid through these orifices results in a pressure gradient across the inside and outside walls of the cylinder. Fluid control devices are a critical component in blow-off valves where they operate as a constant and unregulated throttle device for the delivery of steam into a condenser of a power plant.
These fluid control valves or, more specifically, throttle devices, may also be utilized as a muffler in steam escape devices or in fluid disposal systems. When utilized in fluid control valves, the hollow cylinder of the throttle device often serves as a guide tube for a piston body that reciprocates within the hollow cylinder. The reciprocating piston alternately covers and uncovers a variable quantity of the orifices such that the flow rate of high-pressure fluid passing through the orifices may be regulated. The energy of the high-pressure fluid is reduced as it exits the orifices. A further drop in the pressure of the fluid occurs downstream of the throttle device at a valve seat of the fluid control valve. Yet another pressure drop of the fluid may occur downstream of the fluid control valve if a second, similarly configured hollow cylinder is included.
If the fluid flowing through the fluid control valve is in liquid form, then a high velocity of flow in localized areas of the fluid control valve may reduce the pressure of the liquid to a point which is below the vapor pressure of the liquid. The vapor pressure of a liquid is the pressure at which a portion of the liquid transitions or evaporates into a vapor, forming vapor bubbles. In a control valve, such a reduction in pressure to the level of the vapor pressure may occur downstream of an orifice screen or downstream of the valve seat. The ensuing phase transition of the liquid produces vapor or steam that has a much higher specific volume relative to the specific volume of the fluid when in liquid form. The specific volume is the volume of a substance per unit mass, and may also be defined as the reciprocal of the density of a substance.
The production of vapor from the liquid results in the production of the vapor bubbles. Because the pressure of the flowing liquid will eventually increase in a convergence zone located downstream of the localized area of low pressure, the vapor bubbles will eventually collapse under the increased pressure. The collapsing vapor bubbles at the convergence zone results in very high localized mass accelerations of the fluid, creating the risk of erosion or cavitation damage to walls or diffusers that may come into contact with the flowing fluid. Furthermore, pressure waves resulting from the phase changes of the liquid may have a detrimental effect on nearby fluid control components.
As can be seen, the velocity of the flowing fluid as it moves through the fluid control valve is a controlling factor in the useful life of the fluid control valve. The impact on the useful life of the fluid control valve is due not only to the aforementioned cavitation problems, but also due to the erosion of structural parts of nearby equipment when such equipment is impacted at high speed by droplets of liquid and small foreign particles that may be carried by the flowing fluid. A further disadvantage of a high velocity of the flowing fluid is that the control characteristics of the control valve become unpredictable and irregular. Such irregular control characteristics are the result of discontinuities in the velocity of the fluid. Discontinuities in the velocity of the fluid also create vortices in the convergence zone located behind or downstream of the valve seat of the fluid control valve. Furthermore, high noise levels, structural fatigue, and degradation of the flowing fluid are additional undesirable consequences of high-speed flow.
The above-mentioned problems associated with high-speed flow of fluids through control devices are well known in the art. Attempts to mitigate such harmful effects of high-speed flow in fluid control valves have focused on selecting and developing alloys having suitable mechanical properties. By fabricating control devices out of certain metal alloys, the useful life of control devices can be increased. However, other problems, such as the formation of harmful pressure waves, are unaffected by the choice of material. The prior art includes throttle devices that avoid the problems associated with high-speed fluid flow by incorporating configurations in the fluid control valve that may reduce the high pressures levels of the fluid. Regardless of the various alternative solutions directed towards solving the problems associated with high-speed fluid flow, many fluid control valves still utilize conventional throttle devices because of the increased complexity and the associated high costs of alternative solutions.
Several prior art patents propose solutions to the problems associated with high-speed fluid flow. One such solution is a throttle device wherein the fluid flow is partitioned into a multitude of individual flow tubes in order to effect a reduction in the energy of the flowing fluid. The reduction in the energy is brought about by an arrangement wherein the flow is directed through a plurality of channels. Each of the channels has a high aspect ratio, defined as the ratio of the channel length to the channel diameter. The channels are formed by stacking grooved plates or screens in back-to-back arrangement. The high aspect ratio channels induce a high level of viscous friction within the fluid that is flowing through the channels. The high level of viscous friction effects a reduction in the pressure of the fluid without increasing the velocity of the flow. Each of the channels may define a tortuous flow path defining a number of sudden directional changes in the fluid flow. In such tortuous flow paths, the amount of viscous friction within the fluid may be increased such that the pressure of the fluid may be further reduced.
Although the partitioning of the fluid into channels may be effective in reducing the energy of the fluid, devices incorporating individual channels suffer several drawbacks. One drawback is that the stacking of individual plates or screens is necessarily complex because a large number of plates or screens are needed in order to provide a sufficiently large cross-sectional area of flow necessary for high-flow-rate industrial applications. The cost of manufacturing and assembling the individual plates is proportional to the high quantity required for a single throttle device. Thus, a device requiring a high cross-sectional flow area may be prohibitively expensive. A second drawback is that the control characteristics of such a device are not gradual, as is desired, but rather are incremental or stepwise. This is due to the incremental blocking and unblocking of the channels by a control piston sliding within the fluid control valve. Ideally, it desirable for the fluid control valve to gradually increase and decrease the flow rate through the channels with a high degree of refinement such that the rate of flow may be more precisely regulated.
The prior art includes other throttle devices that are comprised of a single cylinder or several concentric cylinders, with each cylinder being equipped with a multitude of radially disposed holes. However these prior art throttle devices may be even less effective that throttle devices fabricated from stacks of plates. Furthermore, the manufacturing costs of throttle devices comprising concentric cylinders increases in direct proportion to the level of pressure reduction that is desired due in part to the fact that the diameter of the holes affects the noise level of the throttle device. Smaller holes in the cylinders correspond to a higher frequency of noise that is produced by fluid exiting the holes. In an effort to shift a large portion of the noise out of the humanly audible frequency range, the diameter of the holes is reduced. However, in order to constrain the pressure of the passing fluid within a given range, the required number of holes in the cylinders is inversely proportional to the diameter of the holes. Thus, for relatively small hole diameters, a relatively high quantity of holes is required for a given rate of flow. The high quantity of holes required in the cylinder results in increased complexity and higher production costs.
As can be seen, there exists a need for a throttle device that is capable of controlling and throttling high fluid pressures. Additionally, there exists a need for a throttle device that is configured for facilitating a reduction in the pressure of a fluid flowing through a fluid control device. Furthermore, there exists a need for a throttle device that is of a simple construction and which is inexpensive to manufacture.